The Bcl-6 transcriptional repressor is the most commonly mutated oncoprotein in B-cell lymphomas. Mutations of its regulatory regions occur in approximately 50% of patients, leading to deregulated expression of Bcl-6 with consequent differentiation block and aberrant cell survival. The N-terminal BTB/POZ domain of Bcl-6 is required for its transcriptional functions. By X-ray crystallography we found that the Bcl-6 BTB domain binds directly to its co-repressor partner proteins through a non-conserved "lateral groove" motif. The reciprocal surface is provided by a short sequence present in the SMRT, N-CoR and BCoR corepressors. Point mutations in this "COW' motif cause loss of physical and functional interactions between corepressors and Bcl-6 in vitro and in vivo. This could be of major functional relevance for Bcl-6, since corepressor interaction is required for this protein to mediate transcriptional repression. We hypothesize that the lateral groove plays a critical role in Bcl-6 dependent repression. Therefore, we designed peptide inhibitors that prevent co-repressor interaction with Bcl-6. These reagents will be used to "conditionally knock out" the contribution of the SMRT/N-CoRJBCoR co-repressors to silencing by Bcl-6. This will allow us to determine the relative contribution of these corepressors to the genomic and epigenomic transcriptional mechanisms of Bcl-6. Such results can be obtained by combining high-throughput chromatin structure analysis by ChIP on chip and differential methylation hybridization with standard expression arrays. Validation through specific studies on independent loci is also required. Finally, we predict that lateral groove blockade of Bcl-6 will counteract the oncogenic activity of Bcl-6 and constitutes a novel and specific transcription therapy approach for B-cell lymphomas. We will assess the ability of lateral groove blockade to reverse the malignant phenotype of B-cell lymphoma cells as well as its capacity to alter the course of disease in pre-clinical animal models of this disease. Collectively, these results will reveal novel and important insights into the mechanism of action of Bcl-6 transcriptional repression, as well as provide the basis for a novel and specific molecular therapeutic modality for patients with B-cell lymphomas.